replace immutables.Map with immutabledict (#461)
* replace immutables.Map with immutabledict
* Use Mapping rather than any concrete type in public type annotations
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Co-authored-by: Andreas Kloeckner <inform@tiker.net>
* replace immutables.Map with immutabledict
* Use Mapping rather than any concrete type in public type annotations
---------
Co-authored-by: Andreas Kloeckner <inform@tiker.net>